This New York Times article on the Rove case is typically clear as mud, but after reading it several times and consulting with a handful of liberal intellectuals, I've gained new respect for Matt Cooper. Basically, he got fed up with Rove's lawyer lying to the press, and figured that combined with the waiver he'd previously received and the emphasis Luskin placed on it, was enough.
In other words, Rove's lawyer, acting as an agent of Rove, mounted a too extreme PR campaign on behalf of his client, and sufficient deceptive remarks led Cooper to say fuck it. Luskin thought Cooper wouldn't testify no matter what he said, and he was wrong.
Good for Cooper.